Electric arc equipment

ABSTRACT

The invention relates to electric arc processes particularly for underwater use. Electric arc processes effected under water ordinarily operate at a voltage of 20 or 30 volts, with an open circuit of e.g. 50 volts, and are at a level that does not pose a serious risk to the diver. Such as plasma arc cutting has typical voltages of 100-200 volts, and an open circuit voltage of 200-400 volts, and can put a diver at considerable risk of electric shocks at a level that can be life threatening. The object of the invention is to avoid such risks, an objective met by electric arc equipment comprising an earthed safety electrode strategically positioned close to the torch of the electric arc equipment, to create the closest earth to which current could flow, but without the striking of an arc between a torch electrode and the earthed safety electrode.

[0001] This invention relates to electric arc processes, and isparticularly, but not necessarily exclusively, concerned with suchprocesses as are used underwater.

[0002] Various electric arc processes are used underwater, by divers,mainly for welding and cutting. Most required the application of an opencircuit voltage somewhat higher than the operating arc voltage. Thus atypical welding arc voltage of, say, 20 or 30, volts may use a supplywith an open circuit voltage of 50 volts or more.

[0003] A code of practice exists which specifies that the maximumexposed voltage that can be used by divers in this situation is 30volts. It then allows that this may not be practical and thereforeso-called safe methods of working have to be used In practice divers whouse this equipment are accustomed to experiencing electric shocks, butas they are, by experience unpleasant rather than life threatening, theyare tolerated as a part of the process.

[0004] Plasma arc cutting, with typical arc voltages in the range100-200 volts and open circuit voltages of 200-400 volts have notgenerally been used by divers underwater as those levels of voltage aremore likely to prove life threatening. Because of the conductive natureof water there are many potential paths for currents to flow through thewater and a divers body, and it is difficult to distinguish between acurrent which is flowing safely, and one which is likely to bedangerous.

[0005] In general terms most arc processes require two electrodes, oneof which is often the workpiece, and the other usually contained withinan item called a torch or electrode holder (hereinafter called “torch”).In order to produce an arc underwater a gas must be supplied to it, orit must be produced “in situ”—e.g. by creating steam, or by the effectof heat on the electrode material. The resulting ionised gas in the arcis highly conductive compared to water.

[0006] Usually the workpiece is connected to the other side of the arcpower supply and so forms the other electrode (transferred arcprocesses). Often in situations above water this is also connected toearth for safety reasons. In the situation underwater the workpiece canfrequently be considered “the earth” since it is likely to influence theelectrical potentials experienced in the water near to it, and to whicha diver might be exposed.

[0007] There could be situations where different floating objects andthe sea bed could be at different potentials and thus the “earth”referred to is a relative one, not “absolute”.

[0008] There is also the situation of the non-transferred arc process,in which the torch contains both electrodes. In this situation it islikely that the workpiece will still be considered as earth in that itis likely to influence the potential in the water nearby. It is oftenthe case, in plasma cutting processes, for the arc to be initiated innon-transferred mode (pilot arc) but for the cutting to take place intransferred mode.

[0009] With normal arc welding processes used underwater the mostdangerous situation is probably when the supply is applied to the torch,but an arc is not struck. In this situation the applied voltage isgenerally at its highest, and the torch can be positioned away from theworkpiece (earth). As is illustrated schematically in FIG. 1 of theaccompanying drawings this opens up the possibility of exposing thediver directly to the open circuit voltage or to a part of it should thediver accidentally come into contact with the workpiece or be positionedbetween the torch and the workpiece.

[0010] The situation with non-transferred process is similar in that thenon-transferred arc in the torch can impart a potential to thesurrounding water, with respect to the “earth”. As is shownschematically in FIG. 2 of the accompanying drawings, this can put atrisk a diver in the water between the torch and the workpiece.

[0011] The object of the present invention is to provide for the safeoperation of plasma arc processes underwater, or any other process wherevoltage must be applied in a conductive medium and hence could give riseto the electric shocking of an operative.

[0012] According to the present invention, electric arc equipmentcomprises an earthed safety electrode strategically positioned close tothe torch of the electric arc equipment, to create the closest earth towhich current could flow, but without the striking of an arc between atorch electrode and the earthed safety electrode.

[0013] In a preferred form of construction, the earthed safety electrodemay be an annular ring or cylinder, concentrically positioned inrelation to the torch, and whereby an arc struck between a torchelectrode and a workpiece, passes through the annular earthed electrode.

[0014] Desirably, the whole of the torch or electrode holder is eitherinsulated or insulated and fitted with an earthed enclosure. In manycases the earthed enclosure may form sufficient of a safety electrodeeven if it does not obviously enclose the arc. However, the closer thesafety electrode is to the point at which potentials are exposed to thewater, the less likely it is that potentials will be experienced in thewater outside the region of the torch. Thus the form of the safetyelectrode is not material to this invention, but rather its position inrelation to the torch electrode and the arc (or other similar process),and to the diver and the workpiece.

[0015] One embodiment of the invention will now be described withreference to the accompanying drawings in which:

[0016] FIG.3 is a side elevation of a torch illustrating the presence ofa safety electrode;

[0017]FIG. 4 is a schematic sectional view of an electrode, safetyelectrode and workpiece in an open circuit condition;

[0018]FIG. 5 corresponds to FIG. 4 but illustrates a transferred arc;and

[0019]FIG. 6 corresponds to FIG. 4 but shows a non-transferred arccondition.

[0020] In FIG. 3 is illustrated a torch 1 with an umbilical cord 2 forconnection to a source of electrical supply. On the torch is a switch 3,and a nozzle 4 from which will emerge an arc struck between an electrodewithin the body and a workpiece 6. Surrounding the nozzle 4 is a ring 5through which a struck arc will pass, the ring serving as a safetyelectrode.

[0021] In common with conventional practice as is illustrated in FIGS. 1and 2, a potential is created between the workpiece and the torch, but,and as is illustrated schematically in FIGS. 4 to 6, the safetyelectrode 5 is connected to the same earth as is the power supply. Asillustrated it is the workpiece 6.

[0022] Therefore, and in the circumstance where power is supplied to thetorch, but an arc not struck as between an electrode 7 and the workpiece6, i.e. an open circuit circumstance as is illustrated in FIG. 4, anypotential imparted to the surrounding water is earthed through thesafety electrode 5, substantially ensuring that a diver will notexperience an electric shock should he/she be positioned between thetorch and the workpiece.

[0023] Similarly, where there is a transferred arc 8 between theelectrode 7 and the workpiece 6, any potential imparted to thesurrounding water is earthed via the safety electrode 5. Also, andshould a diver touch the workpiece whilst an arc is struck, the safetyelectrode 5 constitutes a preferred path to earth, again substantiallyensuring that a diver is not subjected to dangerous shocks.

[0024] Perhaps the most dangerous circumstance facing a diver is wherethere is a non-transferred arc or pilot arc 9 as is illustrated in FIG.6. Here again, any potential imparted to the surrounding water iseffectively earthed, and even if the diver is holding or is in contactwith the workpiece as the pilot arc is brought close enough to theworkpiece for a transferred arc to be established, the safety electrodecontinues to be a preferred earth, safeguarding the diver againstdangerous electric shocks.

[0025] Whilst the schematic drawings show the principle of connectingthe arc electrode to the supply and safety electrode to the workpiece,it will be understood that all winng can be carried by the umbilical 2to the surface, and where the supply earth can be used for the safetyelectrodes.

1. Electric arc equipment comprising an earthed safety electrodestrategically positioned close to the torch of the electric arcequipment, to create the closest earth to which current could flow, butwithout the striking of an arc between a torch electrode and the earthedsafety electrode.
 2. Electric arc equipment as in claim 1, wherein theearthed safety electrode is an annular ring or cylinder, concentricallypositioned in relation to the torch, and whereby an arc struck between atorch electrode and a workpiece, passes through the annular earthedelectrode.
 3. Electric arc equipment as in claim 1, wherein the whole ofthe torch or electrode holder is insulated.
 4. Electric arc equipment asin claim 3, wherein the whole of the torch or electrode holder isinsulated and fitted with an earthed enclosure.
 5. Electric arcequipment as in claim 1, wherein an enclosure for an electrode fromwhich is struck an arc, forms a safety electrode without enclosing thearc.
 6. Electric arc equipment substantially as hereinbefore describedwith reference to the accompanying drawings.